Oil shale, also called oil-forming shale, is a kind of combustible organic rock with high ash and low metamorphic grade, which is derived from algae and unicellular lower eukaryote or remains as a result of the processes of putrefaction and coalification.

The significance of a thin coal within the open water facies association depends on its origin: given its discontinuous nature and lack of underlying roots, it likely represents organic material (vitrain) resulting from coalification of a transported lycopsid log rather than an in-situ histosol (peat).

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